The relative contributions of glycolytic and oxidative energy production pathways of metabolism in rabbit and mouse preimplantation blastocysts have been determined. We have further studied the relationship between these pathways and active transepithelial Na+ transport. Our results show that over 85% of the ATP production arises from mitochondrial respiration. By using amphotericin B to increase the Na+ permeability of the apical (or uterine-facing) membrane of the blastocyst, we have determined that the ratio of ouabain-sensitive Na+ influx to ATP consumption is three. Based on the measurements of ouabain-sensitive Na+ influx across blastocysts incubated in glucose-containing medium, only 6% of the total energy consumption of the embryo is used for active transepithelial Na+ transport.